1. Field of the Invention
This invention relates to angular relationship and information detection in precision positioning, speed and displacement measurement, and the like. In particular, the present invention is applicable to an angular detection method necessary for information reading apparatus.
2. Description of the Related Art
A conventional encoder of this kind comprises a reference scale that includes information relating to positions or angles, and detection means for detecting information relating to positions or angles while performing relative movement with respect to the reference scale. Such encoders are classified into several types according to the reference scale and the detection means; for example, optical encoders, magnetic encoders, capacitance-type encoders and the like.
As an encoder having higher resolution, an encoder which uses an atomic arrangement as the reference scale utilizing the principle of a scanning tunnel microscope (STM) has been proposed (Japanese Patent Application Public Disclosure (Kokai) No. 62-209302 (1987)).
The scanning tunnel microscope utilizes the phenomenon that a tunnel current flows when the distance between a conductive sample and a conductive probe is reduced to about 1 nm while a voltage is applied between them. The tunnel current exponentially changes in accordance with the distance. That is, if the surface of a sample made of a conductive substance is scanned in two dimensions by a pointed probe while maintaining the distance between the probe and the surface of the sample constant, the tunnel current changes in accordance with the atomic arrangement or the shape of projections and recesses of the surface, whereby an image of the surface of the sample can be obtained (Kotai Butsuri (Solid-State Physics), Vol. 22, No. 3, 1987, pp. 176-186).
That is, by using a sample having a regular atomic arrangement or the shape of periodic projections and recesses as a reference scale, and utilizing the phenomenon that if a relative displacement along the direction of the reference scale is produced between the reference scale and the probe, the tunnel current periodically changes in accordance with the displacement, it is possible to provide an encoder having an atomic-order resolution of about a few .ANG..
In the above-described conventional encoder, a crystal lattice of, for example, graphite (HOPG or kish graphite) may be used as the reference scale. The actual STM image of graphite has the shape of a triangle lattice, as shown in FIG. 1(A). Hence, the detected tunnel current changes between waveforms 7a' and 7b' shown in FIG. 1(B) in accordance with the locus of the moving probe (between arrows 7a and 7b in FIG. 1(A)). For example, if the locus of the probe rides on peaks of the crystal lattice, as represented by arrow 7a, a uniform output signal having a large amplitude and an excellent S/N ratio, as represented by waveform 7a', is obtained. On the other hand, if the locus of the probe is inclined with respect to the direction of alignment of the crystal lattice, as represented by arrow 7b, the amplitude of the output signal is large when the locus rides on a peak of the crystal lattice, but is small when the locus crosses a valley of the crystal lattice, as represented by waveform 7b'. Hence, a uniform output signal cannot be obtained, causing errors in the output of the encoder.
When the encoder having the above-described configuration is used, it is desirable that the probe passes along the locus shown by arrow 7a. Actually, however, when a reference scale using a crystal lattice or the like is set in an encoder, it is impossible to visually confirm the crystal orientation. Accordingly, the orientation of the crystal lattice is in most cases inclined with respect to the direction of detection of the amount of movement, causing detection errors.